The present invention relates to a malfunction monitoring system for a secondary air supply system of an internal combustion engine, and more particularly to a malfunction monitoring system for a secondary air supply system in which a secondary air is supplied or delivered from an intake system to an exhaust system, for example, in the case where the rise of the temperature of a catalyst or catalytic converter in a low temperature state is to be promoted in the internal combustion engine or in the case where an unburned gas in the exhaust system is to be burned again in the exhaust system.
In a technique of purifying hydrocarbon HCl, carbon monoxide CO and nitrogen oxide NO.sub.x in an exhaust gas by use of a three-way catalyst provided in an exhaust system of an internal combustion engine, it is general that an oxygen concentration sensor is disposed in the exhaust system on the upstream side of the three-way catalyst and a control of feeding back a signal representing the concentration of oxygen in the exhaust gas to an internal combustion engine control unit is performed in order to control an air-fuel ratio into a target value which is in the vicinity of a stoichiometric air-fuel ratio providing the most efficient purifying ability of the three-way catalyst. The purifying ability of the three-way catalyst is deteriorated in the case where the internal temperature of the catalyst is low. Therefore, a secondary air supply system is known in which a part of an air flowing through an intake system is supplied as a secondary air to an exhaust system to oxidize HC and CO in an exhaust gas so that the temperature of a three-way catalyst is raised by the generated heat of reaction of oxidation.
The term "secondary air" used in the present specification and claims is a technical term frequently used in the relevant field of art. In contrast with a "primary air" or an intake air which passes through a throttle valve and then flows into a cylinder of an engine, the term "secondary air" means an air supplied directly to an exhaust system without passing through a combustion chamber of the engine.
"On-Board Diagnostic Phase II" which is regulations proposed by the Air Resources Board of the State of California, USA imposes an obligation to install a malfunction monitoring apparatus for a secondary air supply system on a vehicle. The flow rate of a secondary air supplied to the exhaust system of an engine provided with an exhaust emission control system and the amounts of components such as nitrogen oxide, hydrocarbon and carbon monoxide in an exhaust gas substantially have a corresponding relationship therebetween. Namely, as the flow rate of the secondary air is higher, the amount of nitrogen oxide or carbon monoxide exhibits a tendency to decrease. Accordingly, when the flow rate of the secondary air is reduced, the amount of harmful exhaust gas component increases. The relationship between the secondary air flow rate and the amount of a component such as nitrogen oxide or carbon monoxide in an exhaust gas can be determined by measurement. The "On-Board Diagnostic Phase II" requires a malfunction monitoring apparatus to judge a secondary air supply system as being faulty in the case where the measured secondary air flow rate of the secondary air supply system decreases down to a secondary air flow rate value which corresponds to 1.5 times as high as a regulated value for exhaust gas emission amount.
One example of a monitoring apparatus for a secondary air supply system has been disclosed by JP-A-4-1444 which is the laid-open publication of a Japanese patent application filed by Toyota Motor Co., Ltd. on Apr. 18, 1990. The disclosed monitoring apparatus monitors the output of an oxygen concentration sensor when a secondary air is supplied to an exhaust pipe. The diagnosis is made in such a manner that the output of the oxygen sensor shows a lean condition (or a condition in which the amount of oxygen is small) if the secondary air supply system is normal and a rich condition (or a condition in which the amount of oxygen is large) if the secondary air supply system is abnormal. In this monitoring apparatus, the judgement as to whether the secondary air flow rate is high or low is only made indirectly on the basis of the output of the oxygen sensor with the actual secondary air flow rate being not directly measured. In this monitoring apparatus, there may be the case where the oxygen sensor output does not necessarily indicate a true malfunction.
The monitoring or diagnosis of a malfunction of the secondary air supply system with no direct measurement of the secondary air flow rate involves the following problems. In the case where another operating parameter of the engine indirectly representing the secondary air flow rate is to be measured as a substitution for the secondary air flow rate, it is necessary to calibrate a relationship between a true secondary air flow rate and the value of that operating parameter to be measured. When taking the aged deterioration of the engine, it may be necessary to periodically make such a calibration during a useful life of the engine. Also, in the case where a phased change in regulated exhaust gas emission amount from TLEV (Transitional Low Emission Vehicle) to LEV (Low Emission Vehicle) is made as in the State of California, the change of a threshold level for a judgement parameter of a malfunction judging device is required for each phased change and the change of a sensor for measuring the judgement parameter and/or the change of engine specification may be required according to circumstances. Such changes will bring about the rise of an engine development cost.
A secondary air supply system provided with a dedicated air flow sensor for directly measuring the flow rate of a secondary air has been disclosed by JP-A-61-58935 which is the laid-open publication of a Japanese patent application filed by Matsuda Co., Ltd. on Aug. 30, 1984. In the disclosed system, the provision of the dedicated secondary air flow sensor in an engine requires that an input gate for the output of the sensor and an A/D converter for the sensor output should additionally be provided in an engine control unit. Also, in the case where the dedicated air flow sensor is attached in a secondary air path which is proximate to the engine body, a device for countering noises will be needed. The above brings about a problem that an engine manufacturing cost is increased and a problem that the engine control unit becomes large in size and complicated.
An engine deterioration judging apparatus provided with a secondary air supply system has been disclosed by JP-A-3-286165 which is the laid-open publication of a Japanese patent application filed by Matsuda Co., Ltd. on Mar. 30, 1990. In the disclosed apparatus, the negative pressure or vacuum of an intake pipe and the flow rate of an intake air are measured and the inferiority of seal of an intake system of an engine is detected on the basis of a judgement level and a relationship between the measured values of the intake manifold vacuum and the intake air flow rate while the judgement level is changed in accordance with a control signal of the secondary air supply system. This publication has no disclosure of the measurement of the flow rate of a secondary air and the judgement as to the malfunction of the secondary air supply system.